There is a pressing need for drugs to prevent ionizing radiation injury, for use in the field by military personnel and emergency responders. Radiation damages blood-forming tissues, resulting in mortality due to infection and hemorrhage (hematopoietic syndrome). At higher doses, the integrity of the gastrointestinal lining is compromised, which causes death due to loss of fluids and minerals and entry of bacteria into the blood (GI syndrome). The vast majority of studies on the acute radiation syndrome (ARS) and countermeasure efficacy have been performed using photon irradiation (gamma and x-rays). However, in the event of a nuclear detonation, personnel will be exposed to lethal levels of neutrons mixed with gamma rays (mixed fields). Neutrons have different mechanisms of injury to cells and tissues compared to photons, with a higher relative biological effectiveness (RBE). Therefore, countermeasure efficacy may be different between gamma and mixed field exposures, depending on the mechanisms of actions of the countermeasure.